1. Field of the Invention
This invention relates to nuclear fuels, and particularly to discrete nuclear fuel particles comprising essentially fissile element compounds that can be embedded in graphite or other moderator materials to provide fuel compacts or fuel elements particularly suitable for gas cooled nuclear reactors.
2. Description of the Prior Art
It has been proposed to produce individually sealed nuclear fuel particles for gas cooled reactors, which particles each comprise a microspherical kernel of a fissionable or fertile element covered completely with a hermetic carbon shell so that fission gases generated during use of the fuel particles in a nuclear reactor will be retained in the hermetic shell and not escape to the gas coolant in the reactor. As a standard it is desired that not in excess of an average one fuel particle in 10,000 rupture or leak radioactive fission gases during usage of the particles at temperatures of the order of 1000.degree. to 1350.degree. C and fast neutron fluences to 5-8 x 10.sup.21 n/cm.sup.2 with full burn-up of 50 percent or more FIMA (fissions per initial metal atom).
To meet these requirements, the art at the present time employs substantially full density kernels of refractory compounds of fissile elements, for example, UO.sub.2, UC, PuO.sub.2 and PuC, with a low density porous overcoating buffer layer of a substantial thickness in the order to provide a sufficient volume of voids to accommodate at reasonable gas pressures the fission gases produced during use of the fuel particles. Thereafter, coatings of carbon and silicon carbide are applied to hermetically seal in the fission gases, the coatings being sufficiently thick and strong enough to withstand rupture under the pressures resulting from the generated fission gases at burn-ups of up to 70 percent FIMA for example. The prior art fissile kernel microspheres has a diameter of from about 100 to 200 microns, and after the buffer and all the other coatings are applied, the fuel kernel occupies only from 3 to 5.5 percent of the total fuel volume.
When embodied in fuel elements these prior art particles, of UO.sub.2 for example, give a fuel loading of from about 100 to 155 mg. of U per cubic centimeter of fuel element. It would be desirable to provide fuel elements with fuel loadings of 250 mg of U/cm.sup.3 and higher.
In Nuclear Technology, Vol. 16, October 1972 on pages 100 to 109 in an article entitled "The Mechanical Design of TRISO-Coated Particle Fuels for the Large HTGR" by T. D. Gulden et al., there is disclosed in Table I on page 104 fuel particles produced in accordance with the prior art wherein kernels of fissile fuel such as UO.sub.2 or UC, of a diameter from 106 to 117 microns were first coated with carbon buffer layers of a density varying from 1.22 to 1.34 and of a thickness of from 43 to 56 microns. Two high density pyrolytic carbon layers and an intermediate silicon carbide layer were then applied. The total coating thicknesses, per kernel ranged from 98 to 123 microns, the final particle diameter being from about 304 to 363 microns. The fuel kernel comprised only from 3.1 percent to 4.3 percent of the total volume of the six batches of coated fuel particles in Table 1 of this article. In these examples, the volume of the kernel itself to that of the volume of the kernel plus its buffer coating comprised from about 16 percent to 19 percent. The remainder of this latter volume is occupied by the buffer coating necessary to provide space for the expected fission gases. Even so, in Batches 4413-7 and 3516-39 some 3.7 percent and 4 percent of their particles exhibited failed silicon carbide coatings.
What appears to be a desirable design of fuel particle whose characteristics were analyzed for stress characteristics is that set forth in FIG. 2 of this article. The kernel had a diameter of 200 microns, the buffer layer was 85 microns thick and of a density of 1.15 g/cm.sup.3, the first dense pyrolytic carbon coating was 25 microns thick, the silicon carbide layer was 25 microns thick and the outer dense pyrolytic carbon coating was 35 microns thick. The kernel of fissile material comprised 15.8 percent of the volume of the buffer coating enclosed sphere, and only 5.1 percent of the total particle volume.
In the Proceedings of the Fourth International Conference held at Geneva 6-16 Sept. 1971 on "Peaceful Uses of Atomic Energy," in volume 4, appear a number of articles describing fuel particle manufacturing and use. On pages 391 to 402 of Vol. 4 a paper by Schumaker et al. entitled "Preparation of Uranium-Plutonium Carbide Fuel Particles," describes the preparation of microspheres of plutonium-uranium carbides.
On pages 415 to 432 of Vol. 4, in a paper entitled "Fuel Elements for High Temperature Reactors" by Aumuller et al., there are described fuel particles wherein buffer layers of from 50 to 70 microns in thickness are applied to kernels. The kernel diameters for the "feed" fuels is from 150 to 250 microns, because as stated on page 418, this size was necessary "to reduce the internal fission gas pressure at a given thickness of the porous layer coating." For breeding fuels, in which the kernels comprised a fertile material as well as some fissionable element, kernel diameters of 400 to 600 microns were feasible.
It is assumed that the 150 to 250 micron feed fuel kernels in this paper had a 50 micron thick buffer or porous layer applied plus either single 120 micron layer or the three 50-30-35 micron thick high density carbon layer, silicon carbide layer and high density carbon layer applied as in Table III, on page 421. In any case, the volume of the kernel is from 3 percent to 7.6 percent of the final feed fuel particle.
In the paper on pages 433 to 447 of Volume 4, "Utilization of the Thorium Cycle in the HTGR" by H. B. Stewart et al., the making and structure of fissile fuel particles is described. In general, these comprise a microspheric kernel of nuclear fuel of 200 microns in diameter with an applied total coating 130 microns in thickness, with the diameter of the final fuel particle being 460 microns -- and the kernel comprises 8.2 percent of the volume of the fuel particle. Because of the higher proportion of non-fissile material in the fertile fuel particles, the total metal loading per cubic centimeter of a mixture of both fertile and fissile fuel particles is a relatively high 0.7 to 0.8 g/cm.sup.3.
In the paper entitled "Development of a Manufacturing Route for Fuel for the Mark III gas-cooled Reactor (HTR)" by R. W. M. D'Eye and T. J. Heal appearing on pages 449 to 458 of Volume 4, there is set forth the desired standard of less than 2 defective particles in 10,000 and the authors state that "ideally not exceed 1 in 10.sup.4 ". This paper describes processes for producing kernels by tumbling UO.sub.2 along with an additive in a drum so as to produce microspherical kernels of a diameter of some 200 microns. These are then coated.